A thermal printer operates by sequentially heating desired patterns of small discrete areas ("pixels") of a thermal medium to produce desired light and dark patterns on the thermal medium. In some instances, the thermal medium can be a thermally sensitive medium which is heated directly, while in other instances, the thermal medium can be a thermal transfer ribbon which is heated to cause a small amount of dyed wax to be transferred to a medium which is not thermally sensitive.
The discrete areas of the thermal medium are heated by a thermal printhead which includes an array of minute, closely spaced resistive dots (or thermal print elements) that can be individually thermally controlled by means of electrical signals. The thermal medium is stepped past the printhead as each desired linear pattern is printed.
To maintain high print quality under all of the possible operating conditions, it is necessary to keep the spacing between the print medium and the printing elements in the printhead very well controlled. If the distance between the print medium and the printhead is too small, the print will be too dark. If the distance between the print medium and the printhead is too large, the print will be too light. If the distance between the print medium and the printhead varies during a printing session, the print darkness will be unacceptably variable.
At present, the spacing between the print medium and the printhead can be adjusted by means of Allen screws or shims. These devices can adjust the spacing between the printhead and the structure of the printer that holds the printhead against the print medium. However, using these mechanisms to adjust the printhead spacing is clumsy, requires special tools (such as an Allen wrench), and makes it difficult to reproduce the printhead spacing which is found to work with a particular print medium.
One particular circumstance under which the ability to make proper and reproducible adjustments of the spacing between the print medium and the printhead is when the printhead must be replaced in the field.
It is therefore desirable to be able to adjust the spacing between the printhead and the print medium in a thermal printer in a convenient and reproducible manner.
It is further desirable to be able to determine the amount of adjustment which the spacing between the printhead and the print medium has experienced.